This invention relates in general to a solid waste disposal and resource recovery process which produces a useful fuel or chemical synthesis gas, and more particularly, to an improvement upon the process disclosed in U.S. Pat. No. 3,729,298, hereinafter referred to as the Anderson process.
Historically, the least expensive method for disposing of solid waste has been open dumping. However, unprocessed garbage dumps produce severe problems of ground water pollution through leaching, loss of land value, fire hazards and rodent infestation. A more acceptable method, sanitary landfilling has reduced these problems by composting and covering the garbage with earth. Nevertheless, in large metropolitan areas, this practice has become increasingly unattractive as acceptable sites become more scarce. Both of these methods have been supplemented by incinerating the waste before landfilling. While conventional incineration provides significant reductions in the volume of the refuse and some alleviation of the pollution caused by leaching, it introduces new environmental problems such as air pollution, and though volume reductions of 80 to 90 percent are possible, the residue or ash is not biologically inactive and therefore landfilling is still required. Furthermore, resource recovery from conventional incineration tends to be minimal.
One solution to the above problems is disclosed and claimed in U.S. Pat. No. 3,729,298; the disclosure of which is incorporated herein by reference. In summary, the Anderson process disclosed in said patent comprises feeding refuse into the top, and oxygen into the base of a vertical shaft furnace. The furnace (or converter) can be described in terms of having three functional zones; a drying zone at the top, a thermal decomposition or pyrolysis zone in the middle, and a combustion and melting zone (or hearth) at the base. These zones are not clearly distinct; that is, there is no sharp line separating them. As the refuse descends in the furnace, it is first dried by the hot gas which rise through the furnace and then pyrolysed. Pyrolysis is a process whereby organic matter in the refuse is decomposed and thermally cracked in an oxygen-deficient atmosphere with the generation of a CO, H.sub.2 and a char like material. As the refuse moves down through the pyrolysis zone, it is converted to volatile materials which rise and char which descends into the combustion zone. There it is combusted with oxygen, causing the generation of carbon monoxide and carbon dioxide which produce the heat required to melt the inorganic solids in the refuse, such as glass and metal. The molten slag is continuously tapped from the converter, and quenched in a water bath. A gas containing at least 50% (on a dry basis) of a CO and H.sub.2 mixture is discharged from the top of the furnace. Following cleanup, the gas is ready for use as a medium BTU fuel gas or for chemical synthesis.
With natural resources becoming increasingly scarce, there has been an increasing demand to recover them from waste material. It is well known that shredding of refuse prior to further processing is necessary if ferrous metals as well as other material such as tin, aluminum or glass is to be recovered from the refuse by any practical and economical technique. Such recovery processes make use of magnetic fields, electric fields or air classification to separate various components of the shredded refuse. The degree of shredding will vary considerably depending on the nature of the separation process used, as well as the resource to be recovered from the refuse. Although ferrous metal is among the easiest to recover from shredded refuse by magnetic separation, it cannot easily or economically be extracted and purified from the slag-metal residue produced by the Anderson process.
It has been found that when shredded refuse is fed into a shaft furnace and processed in accordance with the Anderson process, it tends to compact so tightly as to restrict the flow of gases through the shaft required for proper functioning of the process. These problems become particularly acute if the furnace is run over a long period of time. One of the problems caused by packing of shredded refuse is that the gas rising from the hearth does not flow uniformly through the entire cross-section of the refuse bed and tends instead to be restricted to a few passages. These passages become enlarged as pyrolysis of the refuse and oxidation of the resultant char occur, eventually leading to the formation of one large channel through which most of the gases then flow. Such channeling reduces the efficiency of the process considerably, since the hot gases from the hearth passing up through the channel have insufficient time and surface contact area to transfer the heat necessary for the gassification, pyrolysis and drying processes to take place. Consequently, the gases leave the top of the furnace at a high temperature, resulting in lower thermal efficiency, an increase in oxygen consumption and a decrease in the BTU value of the product gas.